This invention relates to an aqueous ferrous sulfate waste conversion process, and more particularly to the treatment of such waste for obtaining water effluent that meets economically today's stringent quality standards.
Such aqueous wastes often are effluvia from manufacturing plants practicing the so-called "sulfate process" for making pigmentary titanium dioxide. In such process titaniferous ore, slag, and mixtures of same initially are digested with sulfuric acid and, after a series of operations, pigmentary TiO.sub.2 is recovered. Some aqueous mine wastes also can be candidates for the process; usually they are very dilute in ferrous sulfate (parts-per-million range), so they represent a marginal operation at best. Ferrous sulfate-containing waste pickle liquors from the etching of steel usually are much richer in ferrous sulfate than is a typical moderately dilute (about 0.1-3.5% by weight ferrous sulfate) composite stream from a TiO.sub.2 sulfate process. Such pickle liquors often contain 10-20% ferrous sulfate, and initially the sludging reaction in accordance with this invention can be much faster because of this.
Patent Class 210 sets forth a number of previously-proposed treating methods broadly related to the instant one. Practical, sustained commercial operation in accordance with such methods appears now to be nonexistent for pickle liquors and mine wastes, and is even fairly rare in the pigmentary TiO.sub.2 industry.
The instant process has the advantage over prior suggestions by being more efficient or more economical either in facilities, utilities, reactants, or a combination of these elements per unit volume of waste handled. It is adaptable especially to medium scale operations (up to about 1,000 gallons [3,785 liters] per minute) handling wastes containing ferrous sulfate in concentration above a modest parts-per-million range, e.g. about 0.1-3.5%, although such percentage can be higher. While meeting local water quality standards is imperative for such process, there is very little besides possibly byproduct gypsum and carbon dioxide in certain cases that has any market value from such processing. Accordingly, efficiency and economy, with special emphasis on the latter attribute, is expected to be the deciding factor in process selection, providing of course that the aqueous effluent from the process attains quality standards (not substantially above 1 ppm of ferrous iron often is called for).